Currently, a residential owner of an oil burning furnace calls for burner service either when there is no heat or when an odor is noticed. This need for service often results from inefficient oil burners. In addition to no heat or the production of odors, an inefficient burner also produces soot. Sooting results in fouling of the heat exchanger of the boiler or furnace.
The thermal efficiency of residential oil fired heating equipment in service is lower than the efficiencies that can be achieved with the same equipment under ideal conditions. Two primary factors are responsible. First, there is often a failure to adjust burners during equipment installation and servicing for minimum excess air. Second, a deterioration of thermal performance between tune-ups in continuous service occurs due to soot accumulation on the heat exchanger surfaces.
For maximum thermal efficiency oil burners should have their air/fuel ratios adjusted to produce a "trace" smoke level in the flue (a "trace" smoke level is equivalent to a smoke number between 0 and 1 on the Shell/Bacharach Scale). A burner adjusted this way in 15 steady state, however, will have significantly higher smoke levels during routine, cyclic operation. These higher levels are due to three factors:
1. an ignition pressure peak in the combustion chamber, which has been shown to produce increasingly severe smoke peaks as excess air is reduced; PA0 2. after ignition the average temperature in the chimney is lower than in steady state, leading to reduced draft and excess air; and PA0 3. after ignition the combustion chamber walls are still relatively cold, also leading to increased smoke. PA0 1. Service-required signals. In this mode the homeowner or service company would be made aware that smoke production and/or efficiency have degraded to the point where service is required. PA0 2. Steady-state excess-air trim. In this mode the burner would essentially tune itself continuously for maximum efficiency. Excess air would be changed in response to changes in fuel quality, draft, nozzle erosion, etc. to maintain "trace" smoke in steady state.
Additionally, changes in fuel quality between service calls, as well as excess air changes due to weather conditions, might produce a soot problem for burners set with marginal excess air. Service personnel adjust burners to have generous excess air levels to prevent problems which might require a return visit to the home. Unfortunately, this results in relatively poor operating efficiency compared with the maximum level that can be achieved.
Increasing excess air decreases efficiency by increasing the mass flow rate and temperature of the combustion products discarded to the outdoors. To illustrate the magnitude of these effects, assume that a burner is adjusted to 9% CO.sub.2, rather than an optimal level of 12%. This corresponds to 68% excess air versus the optimal level of about 30%. Stack gas temperature would be about 70.degree. F. higher due to the unneeded excess air. The steady state efficiency would be about 6% lower as a result of these two effects. This example assumes that service personnel have the adequate instrumentation to properly adjust the air/fuel ratio and that the adjustments are actually made. In many cases burners are installed without proper adjustment, leading to very high excess air settings with reduced efficiency and/or service problems.
Estimates of the magnitude of the annual degradation in thermal efficiency based on earlier published studies show considerable variation between units. An average degradation of 2% per year has been used. Principal causes of deterioration are seen as fouling of the heat exchanger surfaces by soot, fouling of the oil nozzle, and changes in the air/fuel ratio caused by dust.
The introduction of advanced control systems can increase efficiency and reduce fuel consumption due to both high excess air and heat exchanger fouling. Two basic control modes can be considered for maintaining high efficiency operation:
The service-required signal mode would reduce fuel consumption by reducing operating time in a degraded condition. A control approach for this mode could be as simple as monitoring the stack temperature as an indicator of fouling. The simplicity of this approach offers a great advantage. However, the homeowner is alerted only after the heat exchanger surfaces have become fouled. A control system which alerts the homeowner when the burner has just begun producing high smoke would eliminate the need for disassembly and cleaning of the unit. This mode could be achieved by measuring smoke, gaseous hydrocarbons, carbon monoxide, or flame optical emissions (color). The present invention is directed to a control system in which the flame optical emission is measured.
Optical methods of flame diagnostics have received increasing attention in recent years and offer a practical method of sensing the quality of an oil burner flame. Monitoring the intensity of the broadband emission from the flame has been found to be a very useful indicator of the excess air. Relative to larger, non-residential burner systems, which have variable or two stage firing rates, the application to fixed firing rate residential systems is simpler. After a burner has been serviced the flame brightness should be about the same each time it fires. The flame brightness with variable firing rate burners is a function of the firing rate.
Measurements of the intensity of light emitted from oil burner flames as a function of wavelength are illustrated in FIGS. 1 and 2. The general nature of the light emitted from oil burner flames is illustrated in FIG. 1. The emission can be considered to consist of two primary parts. The first, or dominant part, is the continuum emission which is like a black body curve and is due to emissions from soot particles in the flame. The second part of the spectra has smaller peaks due to emissions from specific gas phase species in the flame (e.g. OH, CO.sub.2, or CO). FIG. 2 shows an example of measured spectral intensity of radiant energy from an oil flame over the ultraviolet (200-400 nm) and a portion of the visible range (&gt;400 nm). The peak centered at 310 nm wavelength is due to emission from OH. The remainder of the emission is the continuum emission.
The brightness or color of an oil burner flame can be used as a measure of the burner air/fuel or flame quality. For burners which operate at a firing rate which is fixed (for example, by nozzle size), the flame brightness or color at a specific air fuel ratio and with the burner operating in steady state should be constant over time. Monitoring flame brightness or color then, can be a useful method of detecting deterioration of the burner performance over time. As used herein, deteriorated performance means increased smoke or increased excess air, which leads to reduced efficiency. Such deterioration can be caused by a fouled nozzle, fouling of the burner intake openings, a chimney restriction, a fouled heat exchanger, or the collapse of the refractory liner in the combustion chamber. Fixed firing rate burner systems meet a variable load by cycling on and off. In residential oil fired heating equipment, burners cycle 6,000 to 10,000 times each year. During each firing cycle the flame color and brightness changes as the combustion chamber walls warm to their steady state value, with the flame brightness lower at start-up than in steady state. This warm-up period varies from 1/2 to 5 minutes and comprises a significant portion of the on-cycle of the burner.
Accordingly, an object of this invention is to provide an advanced burner control system in residential oil burning furnace which alerts the homeowner to inefficient conditions prior to degradation of the furnace.
Another object of this invention is to provide an advanced means for producing a service call to a residential furnace, prior to the time that burner inefficiency causes severe fouling of the heat exchanger of the boiler or furnace.
A further object of this invention is to reduce heat exchanger cleanings and soot spillage into a home.
Yet another object of this invention is to provide a means for continually indicating the quality of the flame emitted by an oil burning furnace, whether the burner is on or off.